SQL Cheat Sheet & Quick Revision Guide (2026 PDF)

⚡ Core Operations & Code Syntax

# SQL Quick Reference Snippet (ANSI SQL / PostgreSQL)
-- 1. Window Function: Top 3 Highest Paid Engineers per Department
WITH RankedSalaries AS (
    SELECT emp_id, emp_name, department, salary_lpa,
           DENSE_RANK() OVER (PARTITION BY department ORDER BY salary_lpa DESC) AS dept_rank
    FROM employees
    WHERE status = 'ACTIVE'
)
SELECT emp_name, department, salary_lpa
FROM RankedSalaries
WHERE dept_rank <= 3;
# O(1) / O(log N) optimized execution path

⚠️ Common Trap: NULL Comparisons with NOT IN vs NOT EXISTS

If a subquery returns even a single NULL row, 'WHERE id NOT IN (SELECT ...)' evaluates to UNKNOWN and returns 0 rows. Always use 'NOT EXISTS' or 'LEFT JOIN ... WHERE key IS NULL' in production SQL.

✅ Performance Insight: Caching & Vectorization

Replacing row-by-row procedural loops with vectorized SIMD array instructions yields 50x–100x speedups in data manipulation.